Metal containers for foods and beverages are generally made from aluminum, tin-free steel, blackplate and tinplate steel, (which is cold rolled steel sheet coated with a thin layer of tin). Because such metal containers are subject to air oxidation or rusting and to corrosion caused during the pasteurization and sterilization of the beer, soda pop, and various food products which are packaged in such containers, organic reactant polymer coating compositions are used to protect the metal from chemical attack and to protect the food products from the deleterious byproducts of chemical attack. Due to their potential contact with food products, such coatings present additional problems of toxicology and taste considerations.
Christenson et al. U.S. Pat. No. 4,097,438, as well as the patent references cited therein, refer to some of the problems of can coating for beverage and food product operations, and disclose water-based coating compositions for use as coatings for metal containers intended for beverage packages. In the Christenson '438 patent, the polymer is made in the absence of a mercaptan so that the polymer in the coating composition will not influence the taste of the beverage product in the polymer coated can. The Christenson U.S. Pat. No. 3,243,139 refers to can coating composition polymers made with mercaptans. Other of the patents listed in the Christenson '438 patent disclose coating composition polymers made in organic solvent media, but none of such patents recite all of the requirements of this invention.
Brown et al. U.S. Pat. No. 4,195,006 refers to Christenson et al. U.S. Pat. No. 3,991,216 which claims metal containers coated with the compositions claimed in Christenson et al. U.S. Pat. No. 4,065,415, among others, but distinguishes the Christenson '216 and '415 patent compositions in that the Brown et al. '006 patent adds 20 to 45 percent of acrylonitrile or methacrylonitrile monomer to its polymer ingredient of its coating composition, and is thus farther removed from the compositions of this invention.
McFadden U.S. Pat. No. 4,272,621 discloses water thinable coatings compositions of aminoalkylated polymers, polyepoxides and water miscible organic solvents made water soluble by acidification with hydrobromic acid or a mixture of hydrobromic acid and hydrochloric acid to provide improved polymer cure rates, but the McFadden '621 patent does not disclose the components of this invention.
An ideal aqueous coating composition, from the perspective of food and beverage manufacturing would meet the following criteria:
1) formulated from FDA-approved materials, PA0 2) fast curing at low temperatures, PA0 3) high solids, PA0 4) clean, color-free, shiny coating, PA0 5) cured coating should provide tack-free surface with frictional characteristics permitting easy movement in automated can-filling equipment, PA0 6) cured coating should be stable when subjected to high-temperature processing utilized to pasteurize or sterilize foods and beverages, PA0 7) cured coating should be stable when subjected to high temperatures in both acid and basic environments, PA0 8) coating should be capable of adhering to a wide variety of surfaces, including printed and decorated can surfaces, PA0 9) coating should exhibit good shelf life under a variety of atmospheric conditions. PA0 1. Check the can coating gloss and appearance vs. a standard (a previously approved standard batch of these compositions). The coated can should have high gloss and clarity, good wetting and adhesion properties equal to the standard. PA0 2. MEK (methyl ethyl ketone) rubs: PA0 3. Mobility: The Altek Coefficient of Friction (COF) test results are taken, PA0 4. 90 minutes @250.degree. F. Process Resistance test PA0 6.0 percent, by weight, styrene PA0 3.2 percent, by weight, ethyl acrylate PA0 13.5 percent, by weight, butyl acrylate PA0 3.0 percent, by weight, methyl acrylate PA0 5.0 percent, by weight, methacrylic acid and PA0 1.5 percent, by weight, N-isobutoxymethyl acrylamide
Those in the art would prefer to have can coating polymer compositions that satisfy all of the required test parameters, instead of just some of them as well as permit faster can coating machinery speeds. Quality Control tests are made on cans coated with these compositions and include:
approximately 50 +/- 10. The actual number is recorded. This test estimates the can coatings degree of cure (reaction). PA1 a. &lt;0.07 after wash coat bake. PA1 b. &lt;0.07 after additional 3 min. @400.degree. F. PMT inside spray coating bake. PA1 c. &lt;0.08 after additional 10 min. @400.degree. F. TOT overbake. PA1 a. in Deionized Water PA1 b. in Deionized Water adjusted to pH 10.5 with sodium hydroxide (NaOH). PA1 Test can coating against a standard. The coating on the test can should exhibit no delamination or loss of adhesion; can may exhibit slight blush equal to standard in both media. PA1 (a) can be applied in accordance with known can coating methods such as by wash coat, spray coat, curtain coat, roll coat, or in Rutherford can coating machines at substantially faster can coating line speeds than is possible with prior can coating compositions, PA1 (b) will not produce an unacceptable color upon curing and will cure faster at lower bake temperatures, approximately 1 minute at 350.degree. F. PMT to 15 seconds at 400.degree. F. PMT (PMT means peak metal temperature), PA1 (c) will be compliant with EPA and FDA regulations in their manufacture and use in can coating operations, and in their association with beverage and food filling operations which follow, to obtain a can coating that upon curing will possess a sufficiently hard polymeric film surface to prevent can rub or contact marks and be sufficiently color-free to make for an acceptably appearing coated can while at the same time said coating will be open or permeable enough to permit the lubricant contained in the coating composition to bloom, migrate or move to the surface of the coating to facilitate a sufficient can mobility property between the coated cans so that the coated cans will move easily relative to each other and the canning equipment without any substantial rub, contact or scratch marks in can processing and beverage and food filling operations, PA1 (a) an amide containing acrylic resin latex, PA1 (b) a melamine/formaldehyde-type cross-linking resin, PA1 (c) a base treated phenol/formaldehyde cross-linking resin, PA1 (d) a polymer coating compatible lubricant, and PA1 (e) a defoamer selected from the group consisting of (1) an alkyl-branched alkynyl diol having from 10 to 20 carbon atoms, and (2) an aliphatic mineral spirits hydrocarbon mixture having from about 10 to 12 isoparaffinic hydrocarbons therein, PA1 (A) from about 10 to about 90 percent by weight of the total solids in the composition of an acrylic polymer latex containing an acrylamide polymer component made by using the following monomeric ingredients: PA1 based upon the total materials added to the acrylic polymer latex component, in a liquid medium selected from the group consisting of water and a mixture of water with an organic liquid solvent, to provide from about 30 to 35 percent by weight of nonvolatile materials (NVM) in said polymer latex component, PA1 (B) from about 1 to about 50 percent by weight of the total solids in the composition of a melamine/formaldehyde source resin, including a methylated melamine/formaldehyde source resin, PA1 (C) from about 0.5 to 50 percent by weight of a base treated phenol/formaldehyde source resin composition, which phenol/formaldehyde resin has a viscosity of 20 seconds maximum using a No. 4 Ford Cup viscosity test method, PA1 (D) from about 0.1 to about 10 percent by weight of a coating polymer compatible lubricant uniformly dispersible in the total coating composition, and PA1 (E) from about 0.1 to about 5 percent by weight based upon the total solids in the composition of a defoamer material selected from the group consisting of (1) an alkyl branched alkynyl diol having from 10 to 20 carbon atoms and (2) an aliphatic mineral spirits liquid, often called petroleum solvent, having C.sub.10 to C.sub.12 -isoparaffinic hydrocarbons, PA1 and one or more C.sub.1 to C.sub.8 -alkyl acrylate or methacrylate esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, amyl acrylate, hexyl acrylate, heptyl acrylate, 2-ethylhexylacrylate, n-oxtyl acrylate, and the like, and PA1 a vinyl aromatic monomer ingredient such as styrene, alpha-methylstyrene, vinyltoluene (a methylstyrene), or the like. PA1 (A) from about 60 to 85 percent by weight of the acrylic polymer latex, PA1 (B) from about 15 to 40 percent by weight of the melamine/formaldehyde source resin component, PA1 (C) from about 0.5 to 20 percent by weight of the phenol/formaldehyde source resin component, PA1 (D) from about 1 to 5 percent by weight of the lubricant component, and PA1 (E) from about 1 to 4 percent by weight of the defoamer component.
It is an object and purpose of this invention to provide thermoset organic polymeric metal can coating compositions of this nature that with only slight compositional variation:
and which cured polymer coating will also have sufficient "process resistance" properties when exposed to the conventional pasteurization and sterilization temperatures and the varied pH conditions ranging from the acid side pH 5 to basic side pH 10.5 associated with waters typically encountered in beverage and food filling operations, without coating degradation to ensure an adequate can appearance and product shelf life. PA2 said can coating compositions containing therein sufficient liquid selected from the group consisting of water and a water/organic liquid solvent mixture to maintain the component (A) to (E) dispersible therein, preferably at a pH between 6.5 and 8.0. PA2 said can coating composition containing therein sufficient liquid selected from the group consisting of water and a water/organic liquid solvent mixture to maintain the components (A) to (E) dispersible therein. It is preferred to maintain the pH of the composition at a pH of from 6.5 to 8.0, more preferably, between 7.0 and 7.6.
It is also an object of this invention to provide beverage and food product metal cans and metal substrate therefore coated with compositions of this invention.